To test the hypothesis that lower plasma glucose concentrations are required to cause hypoglycemic cognitive dysfunction following recent antecedent by hypoglycemia patients with insulin dependent diabetes mellitus (IDDM) are studied on two occasions once with clamped nocturnal (2330 - 0300 hours) hypoglycemia (approx. 2.5 mmol/L) and once with clamped nocturnal euglycemia (approx. 6.1 mmol/L) in random sequence. Hyperinsulinemic stepped hypoglycemic (approx. 6.1, 4.7, 4.2, 3.6, 3.1 and 2.5 mmol/L) clamps are performed the following morning to assess glycemic threshold for hypoglycemic cognitive dysfunction as well as for neuroendocrine and symptomatic responses.